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Oil Plumes May Be More Toxic Than Thought, Scientists Warn
By John Collins Rudolf

Undersea plumes of microscopic oil droplets extending dozens of miles from the BP wellhead may be more toxic to marine microorganisms in the Gulf of Mexico than previously believed, according to preliminary experimental results from Florida researchers.

Scientists from the University of South Florida, working from a research vessel northeast of the wellhead, found oil droplets scattered in sediment along the gulf floor and in the water column, they said in a report on Tuesday. The dispersed oil appeared to be having a toxic effect on bacteria and phytoplankton, a photosynthetic microorganism that serves as a vital food for fish and other marine life.

The findings require further tests to confirm that the toxic effects are being caused by oil and that the oil originated from BP’s well. But if the results hold true, they would add weight to warnings from independent scientists that the heavily dispersed oil in the gulf remains a persistent threat to sea life.

“It would confirm our worst fears — that the oil is dispersed but that it’s still harming the food web,” said Jeffrey Short, chief scientist for the conservation group Oceana and a former oil pollution researcher with the National Oceanic and Atmospheric Administration, who was not involved with the experiments.

David Naar, an oceanographer with the University of South Florida who took part in the study, said the results were disturbing but that it was too early to conclude that the oil plumes — which are at the level of parts per billion in the water, and invisible to the naked eye — are causing widespread ecological damage.

“It’s not like this dark cloud of death that’s killing everything in the water,” Dr. Naar said.

The dispersed oil does not appear to pose a risk of contamination to seafood in gulf waters that have been reopened to fishing and shrimping, Dr. Short said. “As far as the safety of seafood, I’d say we have nothing to worry about,” he said. “The government’s been very aggressive in testing seafood to make sure it’s O.K.”

But the toxicity of even highly dispersed oil on phytoplankton and other microogranisms, if confirmed, may force the federal government and BP to re-evaluate the long-term impacts of the remaining oil in the gulf. This month, federal scientists released a report suggesting that 74 percent of the estimated five million barrels of oil released from the Deepwater Horizon well had been removed from the gulf through human or natural processes, or had been so highly dispersed that it ceased to pose a significant threat to the ecosystem.

The toxic effects observed by the Florida researchers suggest that this highly dispersed oil, though at very low concentrations, may still pose a risk to microscopic marine life, including eggs and larvae of fish and other creatures.

The threat is greatest to spawning grounds in the vicinity of the wellhead, where the plumes are concentrated, with fisheries in the broader Gulf of Mexico unlikely to suffer an impact, Dr. Short said.

One area of particular concern is DeSoto Canyon, a undersea feature northeast of the well, where nutrient-rich waters are driven by ocean currents onto the continental shelf, helping to support commercially valuable fisheries. These currents could mingle clouds of microscopic oil particles with the larvae of fish, shrimp and other marine life.

“Probably the worst-case scenario is if the same currents that are transporting the dispersed oil at depth focus it into the same places that currents concentrate larvae,” Dr. Short said.